A variable geometry mechanism can adjust a gas throat section, for example, at the inlet to a turbine wheel or at the outlet of a compressor wheel. An adjustment to a gas throat section can alter gas flow direction, gas flow velocity, etc., which may impact back pressure, wheel speed, etc.
As an example, GARRETT® VNT® turbochargers include features to adjust exhaust flow at an inlet to a turbine wheel (e.g., to optimize turbine power for a required load). In such a system, movement of vanes towards a minimum or “closed” position can direct exhaust flow more tangentially to the turbine wheel, which, in turn, imparts more energy to the turbine wheel and, consequently, increases compressor boost delivered by a compressor wheel driven by the turbine wheel. Conversely, movement of vanes towards a maximum or “open” position can direct exhaust flow more radially to a turbine wheel, which, in turn, reduces energy to the turbine wheel and, consequently, decreases compressor boost delivered by a compressor wheel driven by the turbine wheel. Moving vanes to a minimum or “closed” position can also restrict flow and create an increased pressure differential or back pressure (e.g., for any of a variety of purposes related to compressor, turbine, compressor and turbine operation, exhaust gas recirculation, etc.).
As to engine operational conditions and geometry adjustments, at low engine speed and small gas flow, movement toward a minimum or closed position can increase turbine power and boost pressure; whereas, at full engine speed and high gas flow, movement toward a maximum or open position can help to avoid turbine wheel overspeed. Where the turbine wheel drives a compressor wheel, adjustments can help maintain or achieve a desired compressor boost pressure. A controller to control adjustments may include a mechanism that responds to compressor pressure through the use of a pressure actuator, an engine management system using a vacuum actuator, etc.
As mentioned, a variable geometry mechanism may include rotatable vanes arranged a radial distance from a rotational axis of a turbine wheel or a compressor wheel (e.g., in a diffuser section) where each vane includes an upstream edge, referred to as a leading edge, and a downstream edge, referred to as a trailing edge. Between a leading edge and a trailing edge, each vane has a pair of smooth airfoil surfaces. Various mechanical mechanisms exist to maintain spacing between adjacent vanes for a minimum or closed position. However, such mechanisms usually involve various components that may experience wear over time, which, in turn, can alter the spacing between adjacent vanes for a minimum or closed position. Such wear can be detrimental, for example, it may give rise to uncertainties impacting performance as well as emissions.
Technologies, techniques, etc., described in various examples herein pertain to vanes for variable geometry mechanisms, which may be applied to turbines, compressors or turbines and compressors.